1. Field of the Invention
This invention relates to a composite ceramic sintered body which has a high strength and high toughness at a high temperature and an excellent heat resistance, oxidation resistance, wear resistance and erosion resistance, and which is homogeneous and isotropic, and further to a process for the production of the body. This composite ceramic sintered body is suitable for use as high temperature structural elements, precise structural elements, cutting tools and the like.
2. Description of the Related Art
Since ceramics have excellent properties such as heat resistance, oxidation resistance, high strength, wear resistance and high rigidity, they have hitherto been researched and developed for use thereof as high temperature structural elements or wear resistant elements. However, in order to put these ceramics in practical use as various elements, higher toughness and strength have been required. Particularly, in recent years, improvement not only in reliability but also toughness of the ceramics has been greatly required.
That is, in recent years, many studies have been made on the utilization of the ceramic sintered bodies as materials for high temperature structural elements such as blades of gas turbines, hot plugs of diesel engines, housings of internal combustion engines, hot forging dies, rotators of turbochargers; and cutting tools, various nozzles, pipes, rockerarm tips, dies and etc. However, there have never been obtained reliable ceramic sintered bodies.
In order to improve the toughness of ceramics, they should have a great energy for fracture. There have been many proposals of giving them such a great fracture energy. According to the proposals, ceramics such as carbides, nitrides and oxides are used as matrices and fibers or whiskers are incorporated into the matrices to form composites. (See G. C. WEI and P. F. BECHER, "Improvements in Mechanical Properties in SiC by Addition of TiC Particles", Journal of the American Ceramic Society, Vol. 67, No. 8, 1984, pp. 571-574; and C. C. SORRELL, V. S. STUBICAN and R. C. BRADT, "Mechanical Properties of ZrC--ZrB.sub.2 and ZrC--TiB.sub.2 Directionally Solidified Eutectics", Journal of the American Ceramic Society, 69 [4], 1986, pp. 317-321.) The fracture energy of the composite ceramics containing fibers or whiskers therein is greater than that of noncomposite ceramics free of such fibers or whiskers, because the fibers or whiskers allow the course of cracks to turn, or prevent the propagation of the cracks. Furthermore, in such ceramics containing fibrous materials such as fibers or whiskers incorporated therein a mechanism whereby energy is absorbed by allowing the fibers or whiskers to be pulled out from the matrices is considered to be working, in addition to the above-mentioned mechanism. Thus, the greater fracture energy or toughness can be given to ceramics by making various mechanisms in ceramics.
However, when high temperature structural elements or precise structural elements are to be made from the composite ceramics, the composite ceramics are not only required to have a high strength and high toughness but also to have an excellent heat resistance, oxidation resistance and homogeneity. In this respect, ceramics developed until now have still been unsatisfactory and contained many problems.
For example, toughened ceramics of ZrO.sub.2 obtained by utilizing the phase transition loses the toughness when it is heated to a high temperature of, for example 1,000.degree. C. or higher.
In most processes for improving toughness by incorporating a metal or its compounds such as carbides, nitrides, borides or silicides as an additive for giving a composite, the composite is inferior in oxidation resistance to a matrix material alone.
Therefore, the matrix material and the additive both should be individually at least excellent in heat resistance and oxidation resistance, and further combinations of the matrix materials and the additives should be selected to achieve higher toughness. However, for example, many materials having an excellent heat resistance and oxidation resistance have physical properties close to each other. Therefore, it is not so effective to give the toughness to ceramics by accumulating internal strains in the ceramics. Thus, the toughness cannot abruptly be improved.
To the contrary, in a fiber-reinforcing process, the fiber shape and strength of a material to be added to a composite have a great effect on the toughness of the composite. Particularly, studies on composite ceramics reinforced with fibers or whiskers and having the heat resistance and oxidation resistance required in recent years are noticed. (See, for example, S. T. BULJAN and V. K. SARIN, "Silicon Nitride-Based Composites", COMPOSITES, Vol. 18, No. 2, April 1987, pp. 99-106; D. K. SHETTY, M. R. PASUCC and others, "SiC Monofilament-Reinforced Si.sub.3 N.sub.4 Matrix Composites", Ceramic Engineering Science Proceedings, published by the American Ceramic Society, 1985; and T. YAMAMOTO, "Ceramics/Fibers Composites", ELECTRONIC CERAMICS, July 1986, pp. 52-56; and S. T. BULJAN, J. GARY BALDONI and others, American Ceramic Society Bulletin, 66 [2], 1987, pp. 347-352.)
On the other hand, there was reported and example of a one-direction fiber-reinforced composite comprising a glassy matrix reinforced with long carbon fibers or long SiC fibers synthesized from organic silicon compounds. However, these fibers used in the composite are thick and hence the composite is not homogeneous in its properties Furthermore, the composite is tough only in one direction and hence anisotropic. The fibers themselves are poor in heat resistance and oxidation resistance This composite requires to orient the fibers and hence a process for production of the composite is complex Thus, this example has some problems as mentioned above.
There were also reported some examples that whiskers were tried to be used as reinforcing fibers to eliminate the anisotropy and unhomogeneity. (See, for example, Japanese Patent Kokai No. 60-200863; American Ceramic Society Bulletin, 64 [2], 298-304, 1985; or American Ceramic Society Bulletin, 65 [2], 351-56, 1986.)
Japanese Patent Kokai No. 60-200863 discloses a silicon nitride-base ceramics comprising, on the weight basis, 1-30 wt % of at least one in a whisker form selected from the group consisting of SiC, Si.sub.3 N.sub.4, B.sub.4 C and TiB.sub.2,; 1-30 wt % of at least one of carbides and nitrides of Groups IVa and Va elements of the Periodic Table; and 1-10 wt % of Y.sub.2 O.sub.3, the balance being silicon nitride.
In American Ceramic Society Bulletin, 64 [2], 298-304, G. C. WEI, P. F. BECHER and others showed a highly tough silicon carbide whiskers-reinforced alumina ceramic composite as a combination having an excellent heat resistance and oxidation resistance, in which the whiskers were added in an amount of 10 to 30 vol %. However, in all the examples the toughness (K.sub.IC) has been only improved to be at most 8 to 9 MPam.sup.1/2. This is because the diameter of whiskers used is small, which is less effective in preventing the propagation of cracks. If the amount of such whiskers added is too great, then the resultant sintered composite is hard to be densified and, therefore, the added amount is limitative.
In American Ceramic Society Bulletin, 65 [2], 351-56, 1986, P. D. Shallek and J. J. Petrovic showed that the addition of silicon carbide whiskers in an amount of 10 to 30 vol % produced a composite sintered body having a fractural toughness (K.sub.IC) of 10 to 10.5 MPam.sup.1/2. However, the diameter of the whiskers used in this example was 3 to 10 .mu.m, which is much thicker than that of commercially available whiskers, i.e., 0.1 to 5 .mu.m. Such thick whiskers are hardly commercially available. In examples previously reported, on the other hand, whisker-reinforced silicon nitride composite sintered bodies have a fractural toughness (K.sub.IC) of at most 7 to 8 MPam.sup.1/2. Reasons therefor are that particularly in a silicon carbide-reinforced composite sintered body using silicon nitride as a matrix the strength of the silicon carbide is lowered since the sintering temperature of the silicon nitride matrix is high, that the smaller diameter of the whiskers added reduces such effects as mentioned above in preventing the propagation of cracks, and that the amount of the whiskers added is limitative since greater amount of the whiskers added makes it hard to densify the sintered body.
Thus, such prior art techniques as mentioned above have not yet obtained satisfactorily homogeneous, isotropic, heat resistant, oxidation resistant and tough composite sintered ceramic bodies which are suitable for structural elements and etc.